Liquid crystal display devices are characterized by the high quality of images, the small thickness, the light weight and the small consumption of electric power and widely used for televisions, personal computers and automobile navigators. In a liquid crystal display device, two polarizer plates are disposed above and below a liquid crystal cell in a manner such that the transmission axes are perpendicular to each other. Images are exhibited on a display by changing orientation of liquid crystal molecules under application of a voltage to the liquid crystal cell. Many of the liquid crystal display devices of the twisted nematic mode have a construction such that the liquid crystal molecules are arranged perpendicularly under application of a voltage to exhibit a dark display. Many of the liquid crystal display devices of the in-plane switching mode have a construction such that the liquid crystal molecules are oriented in a specific direction in the absence of a voltage, and the direction of the orientation is rotated by 45 degrees under application of a voltage to exhibit a white display.
In a liquid crystal display device in which the transmission axes of two polarizer plates are arranged in a manner such that one of the transmission axes is in the vertical direction, the other is in the horizontal direction, and the two axes are perpendicular to each other, a sufficient contrast can be obtained when the image display is observed in the horizontal and vertical directions. However, when the image display is observed in an oblique direction deviated from the horizontal and vertical directions, the transmission axis of the polarizer plate at the incident side and the transmission axis of the polarizer plate at the output side are not perpendicular to each other in observation. The linear polarized light is not completely shut off, and leak of light arises. A sufficient dark display is not obtained, and the contrast decreases. To overcome this problem, prevention of the decrease in the contrast by adding a means for optical compensation to the liquid crystal display device is attempted.
In Patent Reference 1, (1) a phase contrast film satisfying the relation 0.92≦R40/Re≦1.08 is disclosed, wherein the retardation is represented by Re when a monochromic light having a wavelength of 632.8 nm is incident in the perpendicular direction and the retardation is represented by R40 when a monochromic light having a wavelength of 632.8 nm is obliquely incident at an angle of 40° with respect to the normal of the face of the film.
In Patent Reference 2, (2) a film exhibiting birefringence characterized in that a group of molecules oriented in the direction of the plane of the film and a group of molecules oriented in the direction of the thickness of the film are mixed together, and a process for producing the film exhibiting birefringence characterized in that a shrinking film is adhered to one or both faces of a resin film to form a laminate when the resin film is stretched, and the shrinking force in the direction perpendicular to the direction of the stretching of the film is provided by stretching the laminate under heating, are disclosed.
In Patent Reference 3, (3) a liquid crystal display device in which film (A) transmitting light has at least one light axis or a beam axis in the range of 45° around the normal direction as the reference direction or satisfies the relation nTH−(nMD+nTD)/2>0, wherein the refractive index of the film in the direction of the normal is represented by nTH, the refractive index in the longitudinal direction is represented by nMD and the refractive index in the axial direction is represented by nTD, and at least one sheet of film (A) described above and at least one sheet of uniaxially stretched film (B) comprising a macromolecule having a positive intrinsic birefringence and transmitting light are inserted between a liquid crystal cell and a polarizer plate, is disclosed. As film (A) described above, a laminate having a biaxially or uniaxially stretched film comprising a material having a negative intrinsic birefringence is described.
In Patent Reference 4, a phase contrast plate which comprises a material having a positive intrinsic birefringence and a material having a negative intrinsic birefringence and satisfies the relation Re(450)<Re(550)<Re(650), wherein the retardations at wavelengths of 450 nm, 550 nm and 650 nm are represented by Re(450), Re(550) and Re(650), respectively, is disclosed. In accordance with this reference, the production can be conducted in accordance with simple steps, and a uniform phase contrast can be provided with respect to incident light in the entire range of the visible light.
However, problems are found when a film is produced in accordance with the processes described in the above references. For example, the unevenness in the retardation is great and the efficiency of production is poor in the process described in Patent Reference 1. This process has another problem in that it is difficult that a product having a great size which can be applied to liquid crystal display devices having a great size such as those used for high density television is obtained.
In the process described in Patent Reference 2, it is necessary that the ratio of the stretching and the shrinkage be accurately controlled. This causes a problem in that the production process is complicated, and the efficiency of production is poor.
It may be considered that the film used for liquid crystal devices disclosed in Patent Reference 3 can be produced relatively easily and the phase contrast can be controlled easily, in particular, when a biaxially or uniaxially stretched film having a negative intrinsic birefringence is used as film (A). However, it is actually difficult that a phase contrast film is prepared by stretching a film of a material having a negative intrinsic birefringence such as a vinyl aromatic polymer which is preferable due to a great absolute value of the intrinsic birefringence and excellent transparency. In other words, longitudinal uniaxial stretching by zone heating or transverse uniaxial stretching by a tenter or consecutive biaxial stretching or successive biaxial stretching by a combination of the above uniaxial stretching is necessary for exhibiting the advantageous phase contrast (the retardation) and keeping uniformity of the phase contrast. However, the film tends to be broken during the stretching due to insufficient strength of the material used for the stretching. The exhibition of the desirable phase contrast is suppressed and tends to be fluctuated when the stretching is conducted at a high temperature so that the film is not broken. Therefore, no practically applicable phase contrast film made of a material having a negative intrinsic birefringence and satisfying the relation nTH−(nMD+nTD)/2>0 is existent.
Preparation of a phase contrast film having substantially no in-plane retardation and a refractive index in the direction of thickness greater than the refractive index in the direction of plane (a so-called positive retarder) is made possible by biaxial stretching of a film comprising a material having a negative intrinsic birefringence, and the application to the film compensating the phase contrast of a display device using cholesteric liquid crystals, for example, can be expected. However, the film tends to be broken during stretching due to insufficient strength of the material used for the stretching, and the exhibition of the desirable phase contrast is suppressed and tends to be fluctuated when the stretching is conducted at a high temperature so that the film is not broken. Therefore, no practically applicable film is existent.
The phase contrast plate described in Patent Reference 4 has an insufficient angle of field, and a further improvement is desired.
[Patent Reference 1] Japanese Patent Application Laid-Open No. Heisei 2(1990)-160204
[Patent Reference 2] Japanese Patent Application Laid-Open No. Heisei 5(1993)-157911
[Patent Reference 3] Japanese Patent Application Laid-Open No. Heisei 2(1990)-256023
[Patent Reference 4] Japanese Patent Application Laid-Open No. 2002-40258
The present invention has an object of providing an optical laminate which enables optical compensation in accordance with the mode of the liquid crystal display by the three dimensional control of the refractive index and provides a image display with liquid crystals exhibiting small change in the phase contrast depending on the viewing angle and an optical element and a liquid crystal display device using the optical laminate.